1. Field of the Invention
Aspects of the present invention relate to a proton conductive electrolyte suitable for a fuel cell material and to a fuel cell including the proton conductive electrolyte. More particularly, aspects of the present invention relate to a proton conductive electrolyte that has good proton conductivity and provides good flexibility in a membrane formed using the proton conductive electrolyte. As a result, the proton conductive electrolyte can be used in an electrolyte membrane of a fuel cell or the electrodes thereof, and can provide a solid polymer fuel cell having high current density, high power and long life-time in a dry environment ( relative humidity of 50% or less) at an operating temperature of 100 to 200° C., as well as a fuel cell including the same.
2. Description of the Related Art
Fluorinated polyethylene sulfonic acid is widely used to form electrolyte membranes for fuel cells that are used for salt electrolysis or sea water desalination, water treatment, etc., since the membranes have high proton conductivity and excellent chemical stability. For example, a NAFION® (DuPont) membrane, a FLEMION® (Asahi Glass) membrane, an ACIPLEX® (Asahi Kasei Chemicals) membrane, etc are commercially available. However, these electrolyte membranes contain fluorine, and thus, the membranes have a bad impact on the environment, and are also expensive.
For electrolyte membranes not including fluorine, polystyrene sulfonic acid has been proposed as an ion exchange resin for water treatment or ion exchange membrane, etc., and a sulfonated aromatic polymer has been proposed for a fuel cell (Japanese Laid-open Publication No. hei 11-502245, T. Kobayashi, M. Rikukawa, K. Sanui, N. Ogata, Solid State Ionics, 106, 1998, p. 219). However, the membranes do not have the substantial heat resistance and chemical stability required for a fuel cell.
That is, in terms of power generation efficiency or system efficiency of a fuel cell and the long-term durability of its components, a fuel cell needs to have good electricity generating performance over a long-period of time at an operating temperature of 100 to 200° C. in a dry environment (non-humidified or a relative humidity of 50% or less) . However, when the materials described above are used in an electrolyte membrane for a fuel cell, stable performance cannot be achieved.
In order to improve the electricity generating performance of a fuel cell, the proton conductivity must be increased. This can be done by increasing the concentration of the functional group of an aromatic polymer used in the electrolyte membrane. However, when the concentration of the functional group is higher, the flexibility of the electrolyte membrane is reduced, and the membrane is likely to be fragile.
The inventors of an aspect of the present invention have performed research in order to solve the problems. They have been able to form a membrane where the polyamidic acid maintains flexibility even when a sulfonamide group is incorporated at a high concentration. They have also been able to use polyamidic acid as a precursor and make polyamidic derivatives having high proton conductivity and good flexibility.